1. Field of the Invention
The present invention relates to a device for processing image signals of an original image to reproduce the original image, which is capable of clearly reproducing black characters in case the original image is composed of black characters and color pictures. The device of the present invention is particularly suitable for use in digital color reproduction machines and color facsimile equipment.
2. Description of Background
In color documents comprising therein pictures and characters, the characters are black in most cases. For reproducing black color in color reproduction machines, three color materials, i.e., yellow (Y), magenta (M) and cyan (C), are superimposed on each other. However, perfect black color cannot be obtained unless Y, M and C are perfectly balanced. Also, misalignment of those Y, M and C color materials results in quite indistinct images reproduced, especially, black characters. In order to cope with such a drawback, the prior art utilizes UCR (Under Color Removal) processing in which Y, M and C color materials are replaced by a single black color material in the areas where those three color materials are to be superimposed. Provided that an image reading scanner of color reproduction machines has ideal characteristics, black characters read by the scanner and subjected to color separation into red, green and blue components, could be printed on a paper with a black color material alone using the UCR processing because magnitudes of respective color components should be equal with respect to each other, i.e., R=G=B=O. However, characteristics of the actual scanner are different from those of the ideal one, so the magnitudes of the respective color components cannot be perfectly equal with respect to each other when reading black characters and subjecting them to color separation. An example of the prior art UCR discussed above is reflected by U.S. Pat. No. 4,700,399, wherein the edge intensity is detected by use of a Laplacian filter and the amount of UCR is increased in proportion to the edge intensity of the picture element (pixel). The purpose of this patent is to provide an improved reproducibility of black characters. It must be noted that in this prior art only the control of the UCR amount is performed. In this instance, when the gray balance of R, G and B is incomplete, the color components remain. Thus, the positional shear (slippage) appears between Y, M, C and black. As a consequence, blotting or blurring of color materials for color printing occurs around the black characters so that the image quality of the black picture is deteriorated. Therefore, in spite of using the UCR processing, the black characters are printed on a paper with mixture of black and other color materials.